Binding of Specialty Phosphines to Metals: Synthesis, Structure, and

Apr 30, 2003 - The Pt−P bonds (∼2.26 Å) lie in the shorter range of PtII−P bonds, although the 1J(195Pt31P) coupling (1840 Hz) is quite small. ...
0 downloads 13 Views 122KB Size
Download PDF
2202

Organometallics 2003, 22, 2202-2208

Articles Binding of Specialty Phosphines to Metals: Synthesis, Structure, and Solution Calorimetry of the Phosphirane Complex [PtMe2(iPrBABAR-Phos)2] Ce´cile Laporte, Gilles Frison, and Hansjo¨rg Gru¨tzmacher* Chemistry Department HCI H131, ETH-Ho¨ nggerberg, CH-8093 Zu¨ rich, Switzerland

Anna C. Hillier, William Sommer, and Steven P. Nolan Department of Chemistry, University of New Orleans, New Orleans, Louisiana 70148 Received August 8, 2002

The complex [PtMe2(iPrBABAR-Phos)2] (3) was prepared in a clean and quantitative ligand substitution reaction from [PtMe2(cod)] (1; cod ) η4-1,5-cyclooctadiene) and the phosphirane iPr BABAR-Phos (2). The structure of 3 was determined by X-ray diffraction. The Pt-P bonds (∼2.26 Å) lie in the shorter range of PtII-P bonds, although the 1J(195Pt31P) coupling (1840 Hz) is quite small. The enthalpy for this ligand substitution reaction was measured by solution calorimetry and found to be exothermic by 11.8 kcal/mol, a relatively low exothermicity for a reaction involving a tertiary phosphine in this Pt system. Calculations using density functional theory (DFT) on the B3LYP level were applied using the simplified model reaction [PtH2(cod)] + 2(H2N)PC2H4 f [PtH2{(H2N)PC2H4}2)] + cod, and these also gave a rather low substitution enthalpy (-17 kcal/mol). A charge decomposition analysis (CDA) was performed for Pt(II) and Pt(0) complexes with the simple P-amino phosphirane (H2N)PC2H4 and PH3 as ligands. Contrary to expectations, it is found that the phosphirane acts as a relatively good electron donor, while its electron-acceptor properties are not significantly different from those of other phosphines. The particularly low reaction enthalpy may thus be due to a low directionality of the donor orbitals toward the metal center. Introduction Phosphiranes contain a three-membered heterocycle with at least one phosphorus atom.1-3 The sum of bond angles at the phosphorus center is small (4200 Hz) and is in the upper range of known values, the 195Pt31P coupling in 3 is quite small and falls into the range which is typically observed for Pt(II) complexes with σ-donating alkyl-substituted phosphines. Pt(II) complexes with electron-poor/π-accepting phosphines frequently show larger couplings (>2000 Hz).14,16 On the other hand, the platinum-bonded methyl groups are shifted to rather high frequencies (1H NMR: 0.98 ppm, 2J(195Pt1H) ) 78.2 Hz) which is typical for Pt(II) (13) Nguyen, M. T.; Van Praet, E.; Vanquickenborne, L. G. Inorg. Chem. 1994, 33, 1153. (14) Haar, C. M.; Nolan, S. P.; Marshall, W. J.; Moloy, K. G.; Prock, A.; Giering, W. P. Organometallics 1999, 18, 474-479. (15) Smith, D. C., Jr.; Haar, C. M.; Stevens, E. D.; Nolan, S. P. Organometallics 2000, 19, 1427-1433. (16) Pregosin, P. S. Annu. Rep. NMR Spectrosc. 1986, 17, 285.

Figure 1. Molecular structure of 3. Thermal ellipsoids are drawn at 50% probability; hydrogen atoms were omitted for clarity. Selected bond lengths (Å) and angles (deg) (the two phosphirane ligands show very similar structures, and averaged values are given here): Pt(1)-P(1) ) 2.257(1), Pt(1)-P(1a) ) 2.268(1), Pt(1)-C(19) ) 2.090(4), Pt(1)C(20) ) 2.082(5), P(1/1a)-N(1/1a) ) 1.676(4), P(1/1a)-C(4/ 4a) ) 1.808(4), P(1/1a)-C(5/5a) ) 1.828(4), C(4/4a)-C(5/ 5a) ) 1.571(5), N(1/1a)-C(1/1a) ) 1.485(5), N(1/1a)-C(16/ 16a) ) 1.499(5); P(1)-Pt(1)-P(1a) ) 90.36(4), C(20)Pt(1)-C(19) ) 86.3(2), C(19)-Pt(1)-P(1) ) 91.51(15), C(20)-Pt(1)-P(1a) ) 91.85(14), C(20)-Pt(1)-P(1) ) 177.79(14), C(19)-Pt(1)-P(1a) ) 172.82(14), C(4/4a)-P(1/1a)C(5/5a) ) 51.2(2), N(1/1a)-P(1/1a)-C(4/4a) ) 102.5(2), N(1/ 1a)-P(1/1a)-C(5/5a) ) 103.7(2), N(1/1a)-P(1/1a)-Pt(1) ) 123.0(1), C(4/4a)-Pt(1)-P(1/1a) ) 123.1(2), C(5/5a)-P(1/ 1a)-Pt(1) ) 130.6(1), C(5/5a)-C(4/4a)-P(1/1a) ) 65.1(2), C(4/4a)-C(5/5a)-P(1/1a) ) 63.8(2); Σ°(N) ) 350.3, Σ°(P) ) 257.4.

complexes with electron-poor phosphines such as pyrrolylphosphines, P(pyrl)3.14 Crystals suitable for X-ray analysis were grown from a concentrated CH2Cl2 solution layered with n-hexane. The result of the data refinement (see Table 3 in the Experimental Section) is shown in Figure 1. Because the individual bond lengths in the two coordinated BABAR-Phos ligands in 3 do not vary much, selected averaged bond lengths and angles are given in the caption to Figure 1. In the structure of 3, the two methyl groups and the two BABAR-Phos ligands occupy cis positions in the expected square-planar coordination sphere of the d8 valence electron configured Pt(II) center. For steric reasons, the isopropyl groups at the BABAR-Phos nitrogen center lie on the same side with respect to the central molecular plane, which we also observed for the cationic [Rh(iPrBABAR-Phos)4]+ complex.12 The lengths of the Pt-P bonds in 3 (2.257(1), 2.268(1) Å) are slightly shorter than those in Pt(II) alkylphosphine complexes and approach values observed in complexes with electron-poorer phosphines. The platinum-methyl distances in 3 (2.082(5), 2.090(4) Å) are also in the lower range of previously observed lengths. Note, however, that the variations of the Pt-P and Pt-Me distances in [PtMe2(PR3)2] and [PtMe2(PP)] complexes (PP ) chelating diphosphine) are in general comparatively small (Pt-P ) 2.24-2.34 Å; Pt-Me ) 2.07-2.26 Å) even for sterically and/or electronically rather different phosphines. To this end, we conclude therefore that no particular steric or electronic

2204

Organometallics, Vol. 22, No. 11, 2003

Laporte et al.

Table 1. Comparison of Pertinent Structural Parameters of (i) BTMPBABAR-Phos,11a (ii) [Pt(iPrBABAR-Phos)2(PPh3)2],7 (iii) [Pt(iPrBABAR-Phos)(dvtms)],7,11 (iv) 3, and (v) a tBu-Phosphiranium Cation11 (CN ) Coordination Number)

Chart 1. Depiction of Structures Calculated at the B3LYP Density Functional Level of Theorya

Bond Lengths (Å) Za

a1

a2

b

c

Pt(0), CN 4 Pt(0), CN 3 Pt(II) (3) tBu+

1.850(3) 1.831(5) 1.812(6) 1.808(4) 1.774(3)

1.863(3) 1.856(5) 1.821(7) 1.828(4) 1.774(3)

1.532(4) 1.558(7) 1.576(9) 1.571(5) 1.601(6)

1.738(2) 1.699(4) 1.684(6) 1.676(4) 1.628(4)

Bond Angles (deg) Za

R1

R2

R3

β1

β2

Pt(0), CN 4 Pt(0), CN 3 Pt(II) tBu+

48.8(1) 49.9(2) 51.4(3) 51.2(2) 53.6(2)

66.0(2) 64.2(2) 64.0(3) 65.1(2) 63.2(1)

65.2(2) 65.8(2) 64.6(3) 63.8(2) 63.2(1)

98.8(1) 99.5(2) 101.7(3) 102.5(2) 108.2(2)

99.0(1) 101.0(2) 103.1(3) 103.7(2) 108.2(2)

a

R ) 3,5-bis(trifluoromethyl)phenyl (BTMP).

interactions are reflected in the metric parameters of the platinum coordination sphere. For comparison, the structural parameters of the PC2 cycle in (i) a noncomplexed BABAR-Phos (with 2,5(CF3)C6H2 instead of iPr at the nitrogen atom), (ii) in the four-coordinate Pt(0) complex [Pt(iPrBABAR-Phos)2(PPh3)2],7 (iii) in the three-coordinate Pt(0) complex [Pt(iPrBABAR-Phos)(dvtms)] (dvtms ) divinyltetramethylsiloxane),7,11 (iv) 3, and (v) the tert-butylphosphiranium cation11 are listed in Table 1. As is generally observed for phosphines, the bonds from the phosphorus atom to its neighboring atoms, here the P-C bonds a1 and a2 and the P-N bond c, become shorter when the P lone pair is involved in bonding. Also, the bond angles R1, β1, and β2 are widened. These effects are provoked by increasing electron donation from the phosphorus atom to its bonding partner Z, whereby the positive atomic charge at the P center increases. As the data in Table 1 show, the PfZ donation increases in the order Z ) Pt(0) CN 4 < Pt(0) CN 3 < Pt(II) < tBu+ (CN ) coordination number), placing the complexes between the noncoordinated ligand and the phosphiranium salt in which the PftBu+ interaction can be regarded as pure donation. The structural data show that PfPt electron donation makes a strong contribution to the electronic interaction of BABAR-Phos with platinum centers. If PtfP backdonation plays a dominant role, P-C and P-N bond lengthening is expected (because the transfer of electron density from the metal to the phosphine PX3 occurs in P-X antibonding orbitals). Solution Calorimetry. The substitution reaction shown in Scheme 1 is clean and quantitative at T ) 298 K at reaction times below 1 h. As is detailed in the Experimental Section, an enthalpy ∆Hr of -11.8 ( 0.2 kcal/mol was determined from several experiments,

a Selected bond lengths (Å) and angles are given for 4, 5, 6b, and 9.

which can be compared with substitution enthalpies for a series of structurally related [PtMe2(PR3)2]14 and [PtMe2(PP)]15 complexes that were obtained in a similar manner. In the series with the monodentate phosphines, the highest enthalpy, -34.3 ( 0.2 kcal/mol, was observed for PMe3 as the sterically less demanding electron-donor phosphine and the lowest value, -19.2 ( 0.2 kcal/mol, was obtained for PCy3 (Cy ) cyclohexyl). This small binding energy is certainly caused by the high steric bulk. Note that electron-poor/π-accepting phosphines with “normal” steric requirements such as P(pyrl)3 (pyrl ) pyrrolyl) are also relatively weakly bound; i.e., ∆Hr ) -20.4 ( 0.2 kcal/mol. As expected, for chelating bisphosphines in the series [PtMe2(PP)], slightly higher binding enthalpies are found than with two monodentate phosphines. Also, alkyl-substituted derivatives give larger values (>|-35| kcal/mol) than aryl-substituted ones and with (pyrl)2P-(CH2)2-P(pyr)2 the lowest value (-21.7 kcal/mol) was measured. To this end, BABAR-Phos gives by far the lowest substitution enthalpy measured to date. Quantum-Chemical Calculations. Calculations were performed using B3LYP density functional theory (DFT) in order to gain additional insight into the electronic structure of platinum phosphirane complexes. In particular, we were interested in evaluating the binding quality of a phosphirane toward a Pt(II) center using charge decomposition analysis (CDA) as introduced by Frenking et al.17 As detailed in the Experimental Section, this method corresponds to a “quantified” Dewar-Chatt-Duncanson model and gives information about the amount of LfM donation, d, LrM back-donation, b, and LTM repulsive interactions, r. Second, we wanted to compute the substitution enthalpy for a model reaction close to the one we have investigated in Scheme 1. The model compounds we used for this investigation are depicted in Chart 1. As frequently observed, the distances from the metal to its neighboring (17) Dapprich, S.; Frenking, G. J. Phys. Chem. 1995, 99, 9352.

Binding of Specialty Phosphines to Metals

Organometallics, Vol. 22, No. 11, 2003 2205

Figure 2. Comparison of PH3 and (H2N)PC2H4 rotation barriers relating the conformers 6b and 6d at the B3LYP level of theory. Table 2. Charge Decomposition Analysisa and Interaction Energy Eint (in kcal/mol) for the Phosphines PH3 (8) and (H2N)PC2H4 (9) in Platinum(II) (entries 1 and 2) and Platinum(0) Complexes (entries 3 and 4) Calculated at the B3LYP/II Level entry no.

phosphane/complex

d

b

d/b

r

∆

Eint

1 2 3 4 5 6

PH3 (8)/6b (H2N)PC2H4 (9)/6b PH3 (8)/6c (H2N)PC2H4 (9)/6c PH3 (8)/[Pt(PH3)3] (11)19 (H2N)PC2H4 (9)/[Pt(PH3)2{(H2N)PC2H4}] (12)

0.461 0.598 0.454 0.604 0.382 0.494

0.171 0.203 0.180 0.219 0.229 0.275

2.70 2.95 2.52 2.76 1.67 1.80

-0.379 -0.274 -0.383 -0.272 -0.597 -0.342

0.004 -0.018 0.004 -0.019 0.020 -0.038

32.0 34.1 32.3 34.4 28.3 31.1

a

Donation d, back-donation b, repulsive polarization r, and residual term ∆.

atoms are all slightly too long in the B3LYP calculations in comparison with X-ray crystallographic structures.18 Gross structural features are, however, correctly represented, as the Pt-Pphosphirane bonds in 6b or 7 are found to be slightly shorter than the Pt-PH3 bonds in 5 or 6b and the overall agreement of the calculated structures with experiment is satisfying. In particular, the structural changes within the phosphirane unit upon complexation are reproduced: i.e., the P-C and P-N bonds becoming shorter and the C-C bond of the PC2 cycle becoming longer when 9 is coordinated. Also, the C-P-C angle is widened, while the inner-ring P-C-C angles are narrowed when the phosphirane 9 is coordinated. For the CDA, the mixed [PtH2(PH3)(H2NPC2H4)] complex 6 seemed especially interesting because an internal comparison of the model phosphirane (H2N)PC2H4 (9) versus the phosphine PH3 (8) is possible. Both phosphines are exposed to the strong trans influence of a hydride ligand, while their mutual influence should be small, being bonded in cis positions. Searching on the potential energy surface (PES) for the most stable structure of 6, we found two stable rotational conformers, denoted as 6b and 6d in Figure 2, which differ by only 0.04 kcal/mol. In both conformers, the PC2 cycle adopts a position such that the nitrogen atom of the NH2 group lies almost in the central molecular plane including the phosphorus atoms, hydrogen atoms, and the (18) For Pt(0) phosphine complexes the Pt-P bonds are about 0.030.05 Å longer than the lengths determined experimentally; see: Udin, J.; Dapprich, S.; Frenking, G.; Yates, B. F. Organometallics 1999, 18, 458.

platinum atom. This observation was made also for Pt(0) complexes with monosubstituted phosphines PH2R with R * H.19 However, the barrier for rotation around the Pt-P(phosphirane) bond via the transition state (TS) 6c is very low (0.62 kcal/mol) and is not significantly higher than the rotation barrier for the PH3 ligand in 6b (via TS 6a at 0.12 kcal/mol) or 6d (via TS 6e at 0.38 kcal/mol). This finding can be interpreted in terms of the absence of any particular π-interaction between the phosphirane 9 and the (PH3)PtH2 fragment (neither π-donation or π-acception) in one specific plane of the complex. We discuss here the results of the CDA for the most stable complex, 6b, although the structure 6c is closer to the experimental structure in the sense that the nitrogen atom does not lie in the central plane of the complex. However, the CDA gives very similar results, as indicated in Table 2. The calculated values for donation, d, back-donation, b, and repulsion, r, for the PH3 ligand 8 (entries 1 and 3) and phosphirane ligand 9 are listed in Table 2. In all cases, the residual term ∆ is small, which is a prerequisite to allow a meaningful discussion. For comparison we have also included the data for the tris(phosphine) Pt(0) complex [Pt(PH3)3] (11)19 and the bis(phosphine) phosphirane complex [Pt(PH3)2{(H2N)PC2H4}] (12). In contrast to our expectations, we find that LfM donation, d, increases more than LrM back-donation, b, on going from PH3 (8) to (H2N)PC2H4 (9). As a net effect the d/b ratio for 9 is larger than for 8, and this is (19) Frison, G.; Gru¨tzmacher, H. J. Organomet. Chem. 2002, 643644, 285.

2206

Organometallics, Vol. 22, No. 11, 2003

Chart 2. Substitution Reactions Transforming [PtH2(cod)] (4) Successively into [PtH2(PH3)2] (5) f [PtH2(PH3){(H2N)PC2H4}] (6b) f [PtH2{(H2N)PC2H4}2] (7)a

a Relative energies, enthalpies (in boldface), and free energies (in italics) are given in kcal/mol. Enthalpies and free energies are given at T ) 298 K.

true for Pt(II) and Pt(0). Given that PH3 is an especially bad acceptor, d/b will be even smaller for other phosphines, making the difference between them and 9 even larger.19,20 We find that the d/b ratio is smaller for Pt(0); i.e., back-donation becomes relatively more important with the lower oxidized metal center in accord with qualitative bonding models. Note that also the intrinsic reaction energy, Eint, is higher for the phosphirane 9 than for PH3, regardless of the platinum oxidation state. (Eint corresponds to the interaction energy of the metal fragment MLn with the ligand L when both have already adopted the structure within the complex; i.e., they are electronically and structurally prepared for bonding.) To get a better picture of the thermodynamic substitution energies, we calculated the reactions outlined in Chart 2. The first step in our model reaction sequence is the substitution of the cyclooctadiene (cod) ligand in [PtH2(cod)] (4) by two PH3 ligands, which is modestly exothermic (∆Hr ) -11.18 kcal/mol). The stepwise substitution of each PH3 (8) by (H2N)PC2H4 (9) is slightly exothermic for each step by -2.50 and -3.51 kcal/mol, respectively, leading to an overall reaction enthalpy of about -17 kcal/mol, which is in reasonable accord with the experimentally derived value. The calculations show that phosphirane 9 is a stronger ligand than PH3, which, however, is an especially poorly bonded phosphine. Any other phosphine will be a significantly stronger ligand. Also, the binding energy will increase in the series PH2R < PHR2 < PR3 and therefore especially tertiary phosphines will be more strongly bound to the PtH2 fragment than a phosphirane.20 In accordance with the results from the solution calorimetry and the DFT calculations are the following simple substitution reactions, which we performed in an NMR tube. Addition of 2 equiv of P(OMe)3, P(OPh)3, (20) Frison, G.; Gru¨tzmacher, H.; Frenking, G. Unpublished results.

Laporte et al.

or PPh3 to a CH2Cl2 solution of [PtMe2(iPrBABARPhos)2] (3) rapidly ( 2σ(I)) R indices (all data) largest diff peak and hole

0_75 C38H42N2P2Pt 293(2) K 0.710 73 Å monoclinic P21/n 13.0620(10) 12.5170(10) 20.4639(14) 90 92.990(3) 90 3341.2(4) Å3 4 1.558 Mg/m3 4.324 mm-1 1568 0.5 × 0.5 × 0.2 mm Siemens SMART PLATFORM with CCD detector, graphite monochromator 30 mm ω scans; t ) 10 s full-matrix least squares on F2, SHELXL97 1.81-28.28° -17 e h e 13, -16 e k e 16, -22 e l e 27 28 328 8286 (R(int) ) 0.0726) empirical (SADABS) 8286/0/388 0.999 R1 ) 0.0360, wR2 ) 0.0687 R1 ) 0.0594, wR2 ) 0.0715 2.169 and -1.823 e Å-3

high-vacuum or Schlenk techniques, or in a MBraun glovebox containing less than 1 ppm of oxygen and water. Only materials of high purity as indicated by NMR spectroscopy were used in the calorimetric experiments. The NMR spectra were recorded on a Bruker DPX 250 MHz or DPX 300 MHz spectrometer. 1H and 13C chemical shifts are calibrated against the solvent signal (CDCl3, 1H NMR 7.27 ppm, 13C NMR 77.23 ppm; CD2Cl2, 1H NMR 5.32 ppm, 13C NMR 54.00 ppm). 31P chemical shifts are calibrated against 85% H3PO4 as an external standard. Calorimetric measurements were performed using a Calvet calorimeter (Setaram C-80), which was periodically calibrated using the Tris reaction34 or the enthalpy of solution of KCl in water.35 The calorimeter has been previously described.36,37 The experimental enthalpies for these two standard reactions compared very closely to literature values. The phosphine ligand used in this study was prepared by the literature method.7 (b) NMR Titrations. Prior to every set of calorimetric experiments involving a ligand, a precisely measured amount ((0.1 mg) of [PtMe2(cod)] (1) was placed in an NMR tube along with CD2Cl2 and 2.1 equiv of ligand. Both 1H and 31P{1H} NMR spectra were measured within 1 h of mixing; both indicated the reactions were clean and quantitative. These conditions are necessary for accurate and meaningful calorimetric results and were satisfied for all reactions investigated. (c) Solution Calorimetry. In a representative experiment, the mixing vessels of the Setaram C-80 instrument were (34) Ojelund, G.; Wadso¨, I. Acta Chem. Scand. 1968, 22, 1691-1699. (35) Kilday, M. V. J. Res. Natl. Bur. Stand. (U.S.) 1980, 85, 467481. (36) Nolan, S. P.; Hoff, C. D.; Landrum, J. T. J. Organomet. Chem. 1985, 282, 357-362. (37) Nolan, S. P.; Lopez de la Vega, R.; Hoff, C. D. Inorg. Chem. 1986, 25, 4446-4448.

2208

Organometallics, Vol. 22, No. 11, 2003

cleaned, dried in an oven maintained at 145 °C, and then taken into the glovebox. A sample of [PtMe2(cod)] (1; 16.4 mg, 0.05 mmol) was weighed into the lower vessel, which was closed and sealed with 1.5 mL of mercury. A solution of iPrBABARPhos (2; 30.2 mg, 0.108 mmol) in CH2Cl2 (4 mL) was added, and the remainder of the cell was assembled, removed from the glovebox, and inserted into the calorimeter. The reference vessel was loaded in an identical fashion, with the exception that no platinum complex was added to the lower vessel. After the calorimeter had reached thermal equilibrium at 30.0 °C (ca. 2 h), it was inverted, thereby allowing the reactants to mix. The reaction was considered complete after the calorimeter had once again reached thermal equilibrium (ca. 2 h). Control reactions with Hg and phosphine show no reaction. The enthalpy of ligand substitution (-6.0 ( 0.2 kcal/mol) represents the average of at least three individual calorimetric determinations. The enthalpy of solution of [PtMe2(cod)] (+5.8 ( 0.1 kcal/mol) in CH2Cl2 was determined using identical methodology. This corresponds to a total enthalpy of reaction with all species in solution of -11.8 ( 0.2 kcal/mol. This value can be compared to enthalpy values previously measured for [PtMe2(PR3)2] complexes.14,15 (d) X-ray Crystallography. Single crystals of the complex were obtained by slow diffusion of n-hexane into a concentrated toluene solution. Crystallographic data for the complex were collected on a Siemens SMART PLATFORM instrument with CCD detector and graphite monochromator and are listed in Table 3. The structure of the complex was solved using direct methods and was refined against full matrix (versus F2) with SHELXTL (version 5.0).38 Non-hydrogen atoms were treated anisotropically; hydrogen atoms were refined on calculated positions using the riding model. (e) Synthesis of [PtIIMe2(iPrBABAR-Phos)2]. A 100 mg (0.3 mmol) portion of [(1,2,5,6-η)-1,5-cyclooctadiene]dimethylplatinum(II)39 and 167 mg (0.6 mmol) of iPrBABAR-Phos7 were dissolved in 20 mL of methylene chloride. The resulting yellow (38) Sheldrick, G. M. Acta Crystallogr., Sect. A 1990, A46, 467. Sheldrick, G. M. SHELX 97; Universita¨t Go¨ttingen, Go¨ttingen, Germany, 1997.

Laporte et al. solution was stirred for 1 h at room temperature. The methylene chloride phase was concentrated to 10% of its volume, and hexane was added to precipitate pure complex (187 mg, 87%). 31P NMR (CD2Cl2): δ -69.63 (d, 1JPtP ) 1840 Hz). 1H NMR (CD2Cl2): δ 0.90 (d, 3JHH ) 6.7, CH3 iPr, 6H), 0.98 (m, 2 JPtH ) 78.2 Hz, 3JPH ) 1.6 Hz, CH3, 6H), 2.90 (m, 2JPH + 4JPH ) 5.1 Hz, 3JPtH ) 9.0 Hz, PCHCH, 2H), 4.05 (m, 3JHH ) 6.7 Hz, CH iPr, 1H), 4.98 (m, 3JPH + 6JPH ) 14.3 Hz, 4JPtH ) 11.1 Hz, CH benzyl, 1H), 6.94-7.20 (m, CH aryl, 8H). 13C NMR (CD2Cl2): δ 0.89 (m, CH3, 1C), 3.01 (m, CH3, 1C), 21.60 (m, PCHCH, 2C), 22.32 (m, CH3 iPr, 2C), 50.81 (m, CH iPr, 1C), 59.55 (m, CH benzyl, 2C), 124.26, 126.06, 127.67, 129.72 (s, CH aryl, 8C), 132.24 (m, quat C, 2C), 136.32 (m, quat C, 2C). Anal. Calcd for C38H42N2P2Pt: C, 58.23; H, 5.40. Found: C, 58.34; H, 5.48.

Acknowledgment is made to the ETH Zu¨rich and the Swiss National Science Foundation for support of this work. We are indebted to the Swiss Center for Scientific Computing (CSCS) and the Competence Centre for Computational Chemistry (C4) of the ETH Zu¨rich for providing computer time. The National Science Foundation is gratefully acknowledged for support of the work carried out at UNO. We thank the reviewers for valuable comments. Supporting Information Available: Tables of atomic coordinates and equivalent isotropic displacement parameters, bond distances and angles, anisotropic displacement parameters, and hydrogen coordinates and isotropic displacement parameters for 3 and total energies and complete sets of Cartesian coordinates for the optimized geometries of 4, 5, 6ae, and 7-10. This material is available free of charge via the Internet at http://pubs.acs.org. OM020642Q (39) Costa, E.; Pringle, P. G.; Ravetz, M. Inorg. Synth. 31, 284286; 38, 1623.